In recent years, light-emitting elements using electroluminescence (EL) have been actively researched and developed. In a basic structure of such a light-emitting element, a layer containing a light-emitting material (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting material can be obtained.
Since the above light-emitting element is of a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, the light-emitting element is also effective in reducing the thickness and weight of the display device and increasing the response speed thereof.
In a light-emitting element (e.g., an organic EL element) including an EL layer that contains an organic light-emitting material and is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. Then, the injected electrons and holes recombine, so that the organic material having a light-emitting property is brought into an excited state to provide light emission.
The excited state formed by an organic material can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The statistical generation ratio of S* to T* in the light-emitting element is 1:3. In other words, a light-emitting element including a material emitting phosphorescence has a higher light emission efficiency than a light-emitting element including a material emitting fluorescence. Therefore, light-emitting elements including phosphorescent materials capable of converting a triplet excited state into light emission have been actively developed in recent years.
Among the light-emitting elements including phosphorescent materials, a light-emitting element that emits blue light has not been put into practical use yet because it is difficult to develop a stable material having a high triplet excitation energy level. For this reason, a more stable fluorescent material has been developed for a light-emitting element that emits blue light and a technique of increasing the emission efficiency of the light-emitting element including a fluorescent material has been searched.
As an emission mechanism capable of converting part of a triplet excited state into light emission, triplet-triplet annihilation (TTA) is known. The TTA refers to a process in which, when two triplet excitons approach each other, excitation energy is transferred and spin angular momentum is exchanged to form a singlet exciton.
As compounds in which TTA occurs, anthracene compounds are known. Non-Patent Document 1 discloses that the use of an anthracene compound as a host material in a light-emitting element that emits blue light achieves an external quantum efficiency exceeding 10%. It also discloses that the proportion of a delayed fluorescence component due to TTA in the anthracene compound is approximately 10% of emissive components of the light-emitting element.
Furthermore, tetracene compounds are known as compounds having a high proportion of a delayed fluorescence component due to TTA. Non-Patent Document 2 discloses that the delayed fluorescence component due to TTA in light emission from a tetracene compound accounts for a higher proportion than that for an anthracene compound.
Note that when TTA occurs, the lifetime of a fluorescent material significantly increases (delayed fluorescence is generated) as compared to the case where TTA does not occur. The delayed fluorescence in a light-emitting element can be confirmed by observing the attenuation of light emission after the steady injection of carriers is stopped at a certain point of time. Note that in that case, the spectrum of delayed fluorescence overlaps with the emission spectrum during the steady injection of carriers.